The invention relates to a ceramic composition adapted to form a coating on a metal. The coating is obtained by applying the composition in aqueous slurry. The invention also relates to a method for preparing and applying the composition, the use thereof, and an internal combustion engine exhaust pipe coated with layers of the composition.
In present-day technology, there is a growing need for efficient and formable insulating materials that must have high temperature resistance, excellent thermal insulating power, excellent resistance to thermal shocks, and excellent resistance to chemical corrosion. Such insulating materials are especially attractive to the metal working industry which uses large quantities of sheet-metal, steel and aluminium.
The interest in such materials is especially pronounced in the motor industry (for coating exhaust pipes and engine parts) and in the offshore and ship-building industries (for coatings protecting against fire, corrosion and erosion). Furthermore, these materials are used in combustion plants and processes and in household appliances (kitchen ranges, refrigerators). Above all, however, the motor industry is interested in these materials since motor-cars equipped with catalysts require an appropriate insulation of the exhaust pipe.
It is known from DE-OS 2,751,918 to provide motor-car exhaust pipes with an internal heat-insulating coating consisting of a ceramic slurry with an embedded metal lattice, the interstices of which are filled with the slurry which contains for example sodium silicate, alumina, binder and water. EP-A 101,911 discloses a ceramic coating, in particular for the inner side of an exhaust pipe, said coating containing, besides conventional components, lithium aluminium silicate. US-A 4,248,572 discloses a composition for heat-insulating coatings, said composition comprising ceramic fibres, finely divided silica, an adhesion agent and, optionally, hollow spheres of plastic or ceramic material. It merely appears that the material may be used for insulating furnace walls and furnace parts, and no mention is made of motor-car parts or exhaust pipes.
From GB-A 1.117.950 a ceramic composition for use in bonding an insulating blanket of refractory fibres to a metal surface is known, which composition contains a potassium silicate, barium carbonate and possibly alumina powder. This cement composition thus merely acts as an adhering agent between the insulating blanket and the metal surface, though it is mentioned that a small part of finely chopped ceramic fibres can be included in the cement composition in order to increase the thermal shock resistance of the cement. No mention is made of use in exhaust pipes, the only use stated being on large areas and on steel plates.